1. Field of the Invention
This invention relates to a method and apparatus for adjusting the focus setting of an optical imaging system that acquires an image of a symbol having an encoded character string for the purposes of decoding the symbol and outputting the decoded character string.
2. Description of the Related Art
Encoded symbols and bar codes are typically applied to printed labels, or directly marked on objects for identification and part tracking purposes. The encoded symbol or bar code may contain a lot number or serial number of the part or component, which can be read and decoded using a symbol or bar code reader at various times in an industrial process.
One-dimensional bar codes are often scanned and decoded using a conventional laser-scanner bar code reader. A reader that uses an imaging system is necessary to read and decode more complex symbols, such as two-dimensional symbols or bar codes.
Two-dimensional symbols are used for most direct part marking applications because such symbols can encode a sufficient number of characters while maintaining a relatively compact size. Error correction schemes are typically included in most two-dimensional symbologies, to improve the readability of the symbol. The two-dimensional symbols are often marked directly on the part or component using various methods, depending upon the material composition, part application, and environmental conditions. Common methods include dot peening, laser, and electro-chemical etch.
Symbols marked directly on a part or component are often difficult to read in comparison to symbols printed on labels. An image of a dot-peened, or etched mark often exhibits very low contrast using illumination or image formation systems of most image-based symbology readers. When a symbol is printed on an adhesive label, and applied to a part or component, or the part packaging, a typical image-based symbology reader can provide sufficient read rate. Industry trends, however, suggest that applications of direct part marking are nevertheless increasing.
In a hand-held imaging system reader, the user can improve the read rate by varying or adjusting the distance of the reader to the part bearing the symbol, to effectively change or alter the focal properties of the optical imaging system. A fixed-mount reader that is used to scan objects passing on a conveyor or material handling system must be configured during an initial setup stage. To ensure an optimal read rate of a fixed-mount reader, the imaging system must provide an appropriately focused image, which can be attained by altering the relative position of the reader to the part on the conveyor or material handling system, or by adjusting the focus setting of the optical system of the reader.
An image-based reader has a field of view that increases proportionally in size to the distance of the object from the reader. A fixed-focus reader limits the useful range of the reader. Adjustable focus readers can permit high resolution image acquisition with a small field of view for dense symbols like the data matrix, or low resolution acquisition with a large field of view for large bar codes.
Image-based readers typically employ the use of built-in illumination to provide variable and controlled illumination. Low angle dark field illumination requires a working range where the symbols are placed in close proximity to the reader. Bright field on-axis illumination is typically optimized with an increased relative distance to the reader. A reader having an adjustable focus provides a plurality of working ranges that can be operated with various modes of illumination.
In most low-cost optical readers, the focus setting will be fixed, and the focus setting is attained by adjusting the distance from the object bearing the symbol to the imaging system. To improve the flexibility of imaging systems used to read symbols or bar codes in a variety of working ranges, for example, to provide adequate clearance between the reader and the object due to conveyor and material handling equipment, an adjustable focus capability is desired in image-based readers.